1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses suited to projection.
2. Description of the Related Art
Referring first to FIG. 7, there is shown a commonly available projection type television set which projects an image from the color liquid crystal picture display onto a screen, comprising a white light source 1 issuing collimated light rays, liquid crystal display elements 2 (2a, 2b, 2c), reflection mirrors 3 (3a, 3b), red, green and blue reflection dichroic mirrors 4, 5 and 6 respectively, and a projection lens 7. Under the use of such a construction and arrangement, necessity arises that at least two mirrors such as the reflection mirrors or dichroic mirrors be disposed in each of the spaces (back focal spaces) from the last surface of the projection lens 7 to the liquid crystal display elements. From this reason, a long back focal distance must be made up in any way.
To increase the back focal distance and simplify the structure, it can be considered to construct a zoom lens in such a way that a lens unit having a negative refractive power precedes and is followed by a lens unit having a positive refractive power, or so-called "2-unit" type zoom lens. However, this gives rise to a problem in that the total zooming movement of the second lens unit, when counted from the front, becomes very long, which in turn causes the physical length of the entire system of the zoom lens to increase.